MEMS switches (MEMS=Micro Electromechanical Systems) are used in different areas, in particular, automobile electronics, telecommunications, medical technology, or measurement technology. As a result of their miniaturization, such switching elements, developed as microelectromechanical systems, are also particularly suitable for space travel applications and satellite systems. High frequency-MEMS switches are also used, in particular, in radar systems, satellite communications systems, wireless communications systems, and instrument systems. Examples of this are phased antenna systems and phase shifters for satellite-based radar systems.
High frequency-MEMS switches offer a number of advantages, such as, in particular, a very low current consumption, a good insulation or low stray capacitance, a low insertion dampening or low insertion losses, and low production costs.
DE 10 2004 062 992 A1 discloses a high frequency-MEMS switch. This switch consists of a high resistivity substrate, on whose one side an electrode is provided as a ground electrode. An insulation layer is placed on the opposite side of the substrate; on it, a bent switching element is affixed with the one end, whereas the opposite free end is located in a position bent away from the substrate. If a voltage is applied between the switching element and the ground electrode, then an electrical field is formed essentially between the substrate below the insulation layer and the switching element; the field attracts the bent section of the switching element and is thereby bent straight toward the substrate surface. When the bendable switching element approaches the insulation layer, capacitive coupling takes place between the switching element and a signal conductor which is then opposite it, which represents the actual switching process. In the described, previously known model, this signal conductor is located on the insulation layer; it can, however, also be constructed as an implantation area that is designed below the insulation layer within the substrate, which is contacted toward the outside.
The switching times of such high frequency-MEMS switches are limited by electrical and mechanical effects. For many applications in communications or radar systems, shorter switching times are desirable. As a result of the correlation with the high frequency characteristics of the switches, the mechanical effects cannot be arbitrarily improved. The switching time results thereby from the sum of the mechanical switching time and the electrical switching time, wherein the latter is proportional to the reciprocal value from the product of the lead resistance with the capacitance of the MEMS structure (1(R×C)).
Often, the geometrical dimensions of the HF-MEMS switches and their switching elements (cantilever structures) are optimized, in order to shorten their mechanical switching time. Moreover, overvoltages (actuating voltages, which are significantly above the switching voltage) can be used, so as to further reduce the turn-on time, but not the turn-off time.
The electrical switching time, however, remains unaffected by these measures (namely the optimization of the geometrical dimensions of the HF-MEMS switches and switching elements). Only the electrical turn-on time can also be further reduced by overvoltages, which, however, promotes the accumulation of unwanted charge carriers at the upper edge of the insulator (sticking effect), which can render the MEMS switch unusable. Since the charge carriers must be brought to the underside of the insulation layer, the resistance of the lead, which is decisive for the electrical switching time, predominates due to the substrate resistance and the Schottky contact on the reverse side of the substrate. Typical values for this resistance are on the order of magnitude of 700 kΩ. Typical mechanical switching times are in the range of 8 μs to 100 μs. These vary depending on the applied actuating voltage. If it is too high in comparison to the switching voltage, then the switching times are short, since the cantilever structure is greatly accelerated in the direction of the substrate by a relatively large force. 8 μs represents the value for a high voltage and 100 μs, the value for a low voltage.
Typical electrical switching times, taking into consideration the capacitance on the order of magnitude of 50 pF, are in the range of 30 μs to 90 μs. Also, the electrical switching time fluctuates for small and large actuating voltages of the switch.
From the publications US 2009/0174014 A1 and US 2008/0017489 A1, MEMS high frequency switches with contacts near the surface are known.
The use of doped semiconductor layers under an insulation layer on a high resistivity substrate as the contact electrode is disclosed in the publication by B. Pillans et al., “Schottky contact RF MEMS switch characterization,” Proc. IEEE Int. Microw. Symp., 2007, pages 379-382. In the publication by J. Iannacci et al.: “A general purpose reconfigurable MEMS-based attenuator for Radio Frequency and microwave applications,” EUROCON 2009, IEEE, May 18-23, 2009, pages 1197-1205, a MEMS-high frequency switch is described with a polysilicon electrode doped by implantation under a thin insulator layer.
Buried contacts close to the surface of substrates of high frequency components are known from U.S. Pat. No. 5,628,663. The ohmic contacting of buried, implanted electrodes is also already known from US 2007/0215966 A1.